"Winter is coming." It's not just a saying from Game of Thrones, but something that would definitely worry medieval people at this time of year.
As now, autumn meant both big feasts and battening things down for the winter. Battening required getting in firewood to keep warm during the cold months and long nights ahead (see more here on keeping warm in the Middle Ages). As anyone with a wood stove will tell you, a person can go through an awful lot of wood trying to keep warm in the winter--and modern stoves are far, far more efficient than medieval fireplaces or fire pits.
Autumn also meant stockpiling food. It would be a long time until the new crops came in. Fruits were dried, meat smoked and salted, beans dried. They did not have the canning methods we now take for granted, so no peach halves in syrup. Especially important was making sure that there was enough grain on hand, because bread was the chief source of calories for upper and lower status people alike. And of course the wealthy would want to make sure they had plenty of wine. (Beer could be brewed on a weekly basis.)
But before everything was battened down, it was time for heavy eating (we think of our big Thanksgiving dinner as going back to the Pilgrims in the seventeenth century--it's a lot older than that). All animals, including humans, get hungry as it gets cold. We have thousands of generations of ancestors telling us to fatten up now, because they (the ancestors) "know for a fact" that in four or five months we'll be lucky if we can find some dandelion greens to eat.
November was the month of the medieval pig slaughter; it was even commemorated on churches. Pigs, which had been running wild all summer, fattened up themselves in autumn on fallen acorns (called mast). They were then rounded up, slaughtered, and eaten. Everybody put away as much fresh pork as they could manage, and the rest was salted and smoked (extremely heavily by modern standards) to last the winter.
In a way eating heavily is not just a way to fatten up for winter. It is also a last-hurrah of the good times of the harvest, when food is abundant, and a defiance of shortage and want--you won't get me this time!